The invention relates to a process and device for rotating-code addressing for plasma displays.
The technology of plasma display panels was introduced in the 1960s and culminated in monochrome displays which are highly robust vis-a-vis exterior conditions (temperature, vibrations, etc.), the applications then being essentially military. The progress in the technology made it possible to construct large-sized colour displays used in computer applications (high-resolution workstation, etc.). The end of the 1980s saw the birth of the idea of being able to display video on a plasma panel, involving a refreshing of the display at the rate of 50 images per second. This video timing constraint has still not been fully overcome at the present time, and a number of problems persist.
The term plasma panel describes the state of the gas contained in the panel. Thus, a plasma display panel consists of two glass panes separated by about a hundred microns. During manufacture, this space is filled with a gaseous mixture containing neon and xenon. When this gas is excited electrically, the electrons orbiting the nuclei are extracted and become free. The term xe2x80x9cplasmaxe2x80x9d denotes this gas in the excited state. Electrodes are silk-screen printed on each of the two panes of the panel, line electrodes for one pane and column electrodes for the other pane. The number of line and column electrodes corresponds to the definition of the display panel. Again, during the manufacturing process, a barrier system is set in place which makes it possible physically to delimit the cells of the panel and to limit the phenomena of the diffusing of one colour into another. Each crossover of a column electrode and a line electrode will correspond to a video cell containing a volume of gas. A cell will be referred to as red, green or blue depending on the luminophore deposit with which it will be covered. Since a video pixel is made up of a triplet of cells (one red, one green and one blue), there are therefore three times as many column electrodes as pixels in a line. On the other hand, the number of line electrodes is equal to the number of lines in the panel.
Given this matrix architecture, a potential difference merely needs to be applied to the crossover of a line electrode and a column electrode in order to excite a specific cell and thus obtain, point-wise, a gas in the plasma state. The excitation of the gas is accompanied by the generation of UV which will bombard the red, green or blue luminophores and thus give a red, green or blue illuminated cell.
A line of the plasma panel is addressed as many times as are defined therein sub-scans in the grey level to be transmitted to the pixel (one speaks of grey level for each of the three components R, G, B, this level lying between 0 and 255). The pixel is selected by transmitting a voltage termed a write pulse, by way of a driver, to the whole of the line corresponding to the selected pixel while the information corresponding to the grey-related value of the selected pixel is transmitted in parallel to all the electrodes of the column in which the pixel lies. All the columns are supplied simultaneously, each of them with a value corresponding to the pixel of this column.
With each bit of the grey level information there is associated a time information which therefore corresponds to the bit illumination time or more globally to the time between two writes: a 1 value for the bit of order 4 will thus correspond to the pixel being illuminated for a duration 4 times greater than the illumination corresponding to the bit of order 1. This hold time is defined by the time separating the write cue from an erasure cue. For a grey level coded on n bits, the panel will be scanned n times in order to retranscribe this level, the duration of each of these sub-scans being proportional to the bit which it represents. By integration, the eye converts this xe2x80x9cglobalxe2x80x9d duration corresponding to the n bits into a value of illumination level. Sequential scanning of each of the bits of the binary word is therefore performed by applying a duration proportional to the weight. The addressing time of a pixel, for one bit, is the same irrespective of the weight of this bit, what changes is the illumination hold time for this bit.
All the pixels of a line are addressed simultaneously by a line driver whose load and hence the current which it must deliver depend on the number of pixels illuminated in this line. When changing from one sub-scan to the next, that is to say from one weight to another, the load, and hence the level delivered by the driver, may change, generating overbrightness effects, as explained later.
Given the present-day characteristics of panels (N1 lines) and the time required to address a line (tad), it is only possible to perform 10 sub-scans (n=10) in 20 ms. Since the video is generally coded from 0 to 255, i.e. on 8 bits, 2 extra sub-scans are therefore available. There is known, from the prior art, a transcoding of the 8-bit coding word of the video into a 10-bit coding word supplying the columns and which will be referred to here, in a general manner, as a column control word. This transcoding splits each of the two high-order bits of value 64 and 128 respectively into two sub-scans of weight 32 and two sub-scans of weight 64. Thus, the value 64 or 128 is coded by giving the value 1 to the two sub-scans of weight 32 or the value 1 to the two sub-scans of weight 64 of the column control word, thus distributing the load of the driver over the duration of the frame. However, this transcoding does not satisfactorily resolve the overbrightness effects which still remain harmful.
The object of the invention is to lessen, in a very efficient manner, the overbrightness defect.
The subject of the invention is a process for addressing cells arranged as a matrix array, each cell being situated at the intersection of a line and a column, the array having line inputs and column inputs for displaying grey levels defined by video words making up a digital video signal, the column inputs each receiving a control word for this column corresponding to the video word relating, for this column, to the,addressed line, this word being made up of n bits transmitted sequentially, each bit triggering or not triggering, depending on its state, the selection of the cell of the addressed line and of the corresponding column for a time proportional to the weight of this bit within the word, characterized in that it comprises a step effecting a transcoding of the video words into column control words such that the number of bits of the column control words is greater than that of the video words and such that different column control words are used for coding the same grey level of the video signal.
The invention also relates to a device for addressing a plasma panel for the implementation of the process, comprising a video processing circuit for processing the digital video data received, a correspondence memory for transcoding these data, a video memory for storing the transcoded data, the video memory being linked to column driver circuits in order to control the column addressing of the plasma panel, characterized in that the transcoded data have a greater number of bits than the digital video data received and in that the processing circuit comprises means for differently coding identical values of digital video data received.
By virtue of the invention, the illuminated cells are distributed more homogeneously over the timescale; the same is true for the load of the line drivers of the plasma panel which is thus better distributed so as to attenuate or even eliminate the overbrightness effects. The invention is simple and inexpensive to implement.